In a typical scaling-down/scaling-up procedure for a video image frame, an on-screen resolution technology is employed to process the video image frame by a video system for a real-time display on a screen. Because the number of image frames displayed on the screen per second should be more than 30 to exempt from image delay, the compressed video images are stored in a frame buffer register in a form of a one-way queue, and then processed by the video system for display. One disadvantage thereof is the requirement for a large capacity of frame buffer register able to store considerable data at the same time. Although the image data consisting of a plurality of rows is easy to be compressed or scaled up in the horizontal dimension, it is difficult to be so processed in the vertical dimension. Therefore, the frame buffer register needs to have a memory capacity enough to store the plurality of rows of the image data. For example, when every three rows of image data are to be compressed into one row, the memory size of the frame buffer register should be capable of storing at least three rows of image data at the same time. On the other hand, for a compression rate up to five, i.e. one row from five ones of image data, the memory size of the frame buffer register is required to be capable of storing at least five rows of image data. As a result, the memory size of the frame buffer register has to be large enough for various compression rates, and thus costs a lot. The similar requirement also applies to the scaling-up situation in view of the image quality. Due to an increasing demand of high compressing rate or scaling-up rate and a memory size limitation of the frame buffer register, the scaling-down/scaling-up procedure by means of the on-screen technology will not assure high quality of a displayed image.